There is increasing evidence that the foam cells found in the atheroscleotic reaction are macrophages which are derived from blood borne monocytes and/or smooth muscle cells which have taken on the properties of macrophages. We have developed techniques for the rapid isolation of pure monocytes and have been able to convert them into macrophages in vitro. Using these cells we learned that the widely held notion that cellular cholesterol concentrations are governed primarily by the relative amounts of LDL and HDL is not true. Incubating these cells in high concentrations of normal LDL did not cause the accumulation of cholesteryl esters even when the HDL concentration was low. While the cells did not accumulate cholesteryl esters when exposed to normal LDL, they did degrade LDL at a rate that should have liberated an amount of free cholesterol sufficient to double the cholesterol content of the cells every 24 hours. Such an increase was not observed. However, when the LDL was polymerized or its charge modified either chemically or by the action of normal human platelets there was a dramatic increase in the cholesteryl ester content of the cells. Based on these preliminary studies we propose that LDL must be modified in vivo that leads to cholesteryl ester accumulation in the cells of the arterial wall is mediated by the action of blood platelets. The experiments proposed in this application are designed to test these hypotheses. Other experiments are proposed to further characterize the basic defect in familial hypercholesterolemia. We also will use monospecific antibody to study the regulation of HMG-CoA reductase in human monocytes. In other experiments we will explore the mechanisms regulating cholesterol efflux from human peripheral cells. We believe that these studies will substantially increase our understanding of human lipoprotein and cholesterol metabolism.